They can be hacked: "Not during actual elections — not even
under real-world conditions — but hacked nonetheless. In July
2003, a team of computer scientists from Johns Hopkins University
announced that it had reprogrammed the market-leading Diebold
AccuVote-TS by popping a lock and replacing the memory card."

They are expensive: "Paper and pencils cost relatively
little. They require no maintenance. By contrast, touchscreens
sell for roughly $6000 apiece... 'For every machine that's
purchased,' one expert says, 'there's a picture of a
schoolteacher who wasn't hired.'"

Prone to human error: "In 2004, 4438 votes in Carteret
County, N.C., went missing. The hardware worked just fine, but
local administrators had inadvertently programmed the software to
accept fewer votes than the community required." And many other
examples.

They don't leave physical records: "In a disputed election,
voters [have] little choice but to trust the machines. If tallies
were lost—to malfunction or malfeasance—there was no way to get
them back." Efforts to create paper trails are problematic in
themselves — printers break, and there is plenty of room for
human error.

In today's election, reports have been circling (including
video) of an electronic voting machine that seems to be
glitchy — it only has a tiny area that can select Obama as
president, otherwise a click on Obama's check-box results in a
Romney vote. A report from MSNBC suggests that the machine
has been taken out of use.